why can't i get it on the 3ds?

You may need to update your game at the title screen perhaps.It resolves most similar questions.That's all i know ^^'.

the option isn't there

Weird,i remember that there was an update option on 3DS. Hmmm...I've looked up and i'll recommend you to look up at " yachtclubgames.com/sk-how-to-update/ " and see if you've done any of the followed instructions youself.But otherwise basically:be sure your console is updated,you can go in the eshop then search for "Shovel Knight"/"Shovel Knight:Treasure Trove" and see if theire is a patch/update!

Though,you should get updates normally when starting to play if the console has the wireless connection on. And i doubt it has anything to do with Spotpass though maybe...